It is often desirable to inspect containers, such as aluminum beverage cans, for defects and to identify defective containers before they are filled and sealed. Today's automated processing equipment can fill and seal containers at speeds of approximately 1800 containers per minute. Although manual inspection is possible, it is slow and expensive and provides opportunities for the containers to incur damage during the interim between inspection and processing. Therefore, engineers have developed systems that automatically inspect each container shortly before it is delivered to the processing equipment.
One type of automatic inspection device illuminates the interior of each container as it passes on a conveyor and acquires multiple images of the interior. The device converts each image into pixels and each pixel into a digital value, which is stored in an image buffer. A processor then executes an algorithm that both determines which pixels to inspect for defects and inspects those pixels. When a defect is detected, the device removes the defective container from the conveyor.
Because the processor must execute the lengthy inspection-pixel algorithm for the multiple images of each container, the device cannot inspect containers with the desired pixel resolution while operating at the desired speed. Thus, there is a trade off between inspection resolution and speed. Also, one portion of the interior may have a different reflectivity than another portion. In order to prevent image saturation, each camera's sensitivity is set to accommodate the highest expected reflectivity. Thus, the defect sensitivity of the device for an interior portion having a lower reflectivity may be less than that for a portion having a higher reflectivity. Additionally, different types of containers may have interiors with different average reflectivities. Thus, a technician may have to manually calibrate the sensitivity of each camera between processing runs of different types of containers.